This project is based on our discovery that genetic mutations in molecules that control the programmed death, or apoptosis, of lymphocytes are responsible for the Autoimmune Lymphoproliferative Syndrome (ALPS). ALPS is a disease affecting children that leads to loss of normal lymphocyte homeostasis leading to swollen lymph glands and organs. Because lymphocytes are the primary cell mediating immune reactions, this excess of lymphocytes leads to a pathological autoimmune attack on the patients own tissues. We have identified mutations in a death-inducing cell surface receptor termed Fas (also known as APO-1 or CD95) and in other molecules that regulate apoptosis. We have also identified a new disease entity called, Caspase-8 Deficiency State (CEDS) that is due to a genetic deficiency of caspase-8. This disease involves a loss of apoptotic control and lymphocyte expansion combined with a failure of normal lymphocyte activation through the antigen receptors. The consequence of this is a profound immunodeficiency state and a new insight that caspase-8, heretofore regarded solely as a cell death inducing protease, has a key role in antigen receptor signaling particularly for the induction of a gene regulatory factor called NF-kB. These studies promise to provide new insights into the molecular mechanisms that underlie autoimmune and immunodeficiency disease as well as revealing crucial steps in the pathway of programmed cell death in lymphocytes. In other related studies, we have shown that mutations in Fas and caspase-10 can be co-inherited in ALPS patients. We have also found that mutations in Fas ligand that cause ALPS have dominant-interfering properties thus explaining why such mutant alleles are disease-causing in a heterozygous state. Our work also permits us to address the pathogenic role of apoptosis defects in other immunological diseases such as X-linked proliferative disease or other less well-defined conditions. We are presently studying a class of these patients called ALPS Type III which do not display mutations in the Fas receptor, its ligand (Fas ligand), or caspase-10. We are using a variety of molecular analyses to determine the gene mutation that underlies disease in ALPS Type III. These experiments have been successful in uncovering the molecular basis of a new class of this disease, ALPS type IV. Patients with this disorder have typical clinical features of autoimmunity and abnormal lymphocyte homeostasis that are detected in ALPS, type I and II. however, these patients differ in that they have a strikingly decreased death in response to cytokine withdrawal rather than a defect in death receptor apoptosis. The molecular basis of this disorder is a reduction in the apoptosis protein Bim due to an inherited germline mutation in the N-Ras oncogene. We plan to continue to examine unusual Alps Type III cases to understand their molecular basis. Our guiding principle is that patient specimens from poorly understood diseases can yield valuable insights into disease mechanisms and normal physiology if investigated properly at the molecular level. We have found several new mutations in these unusual patient disorders and are currently characterizing their role in lymphocyte homeostasis and apoptosis. In studying a group of patients with abnormal immune regulation, we focused in the past year on a group of patients that had idiopathic CD4 lymphopenia. These patients had exhibited this condition from birth and had no known external causes of low CD4 T cells such as HIV infection. Our studies led to the the identification of a magnesium channel, termed MagT1, that is critical for the selection of CD4 T cells in the thymus as well as peripheral function of T but not B lymphocytes. The magnesium ion, Mg(2+), is essential for all life as a cofactor for ATP, polyphosphates such as DNA and RNA, chorophyll, and metabolic enzymes, but whether it plays a part in intracellular signalling (as Ca(2+) has been shown to play) was unexplored. we discovered inactivating mutations in the magnesium transporter gene, MAGT1, in a novel X-linked human immunodeficiency characterized by CD4 lymphopenia, severe chronic viral infections, and defective T-lymphocyte activation in the patients we were investigating. We found that a rapid transient Mg(2+) influx is induced by antigen receptor stimulation in normal T cells and by growth factor stimulation in non-lymphoid cells and that this influx is critical for cellular activation. MAGT1 defects abrogate this Mg(2+) influx and decrease the basal level of free Mg(2+) but not the total bound Mg(2+). These alterations lead to impaired responses to antigen receptor engagement, including defective activation of phospholipase C&#947;1 and a markedly impaired Ca(2+) influx in T cells but not B cells. Our observations reveal a likely role for Mg(2+) as an intracellular second messenger coupling cell-surface receptor activation to intracellular effectors and identify MAGT1 as a possible target for novel therapeutics.